Field of the Invention
The invention relates to an antenna that transmits a transmission wave.
Description of the Background Art
A radar apparatus derives target information that represents a position and the like of an object (hereinafter referred to as “target”) by transmitting transmission waves and then receiving reflection waves reflected from the target. A vehicle controller that acquires the target information output from the radar apparatus controls an accelerator and a brake for appropriate vehicle control.
The transmission waves transmitted by the radar apparatus are reflected by various targets existing around a host vehicle on which the radar apparatus is mounted, and the reflected waves are received by the radar apparatus. Examples of types of the targets are “control-required target,” “upper target” and “lower target.”
The type “control-required target” refers to a target that exists on a road on which the host vehicle equipped with the radar apparatus 1 is traveling, such as a preceding vehicle traveling in front of the host vehicle. If the host vehicle moves closer to the control-required target, the host vehicle and the control-required target may collide with each other. Therefore, in a case where the host vehicle gets closer to the control-required target, vehicle control is required, for example, to avoid a collision.
The type “upper target” refers to a target that exists above the road on which the host vehicle is traveling, such as a billboard and a traffic sign. Even in a case where the host vehicle moves closer to the upper target, there is no possibility of a collision of the host vehicle with the upper target. Therefore, if the host vehicle gets closer to the upper target, no vehicle control is required. The type “lower target” refers to a target that exists on the road on which the host vehicle is traveling, such as a manhole and a fallen object. Even in a case where the host vehicle moves closer to the lower target, there is no possibility of a collision of the host vehicle with the lower target. Therefore, if the host vehicle gets closer to the lower target, no vehicle control is required.
Conventionally, a radar apparatus includes three types of antennas: a horizontal antenna, an upper antenna, and a lower antenna. The horizontal antenna transmits transmission waves along a reference transmission axis in substantially parallel to a surface of the road. The upper antenna transmits transmission waves in an obliquely upward direction relative to the reference transmission axis. The lower antenna transmits transmission waves in an obliquely downward direction relative to the reference transmission axis. The reference transmission axis is a virtual transmission axis.
The control-required target, the upper target and the lower target exist in vertically different positions relative to the reference transmission axis. The radar apparatus transmits the transmission waves from the three types of the transmitting antennas in order and derives the vertical positions of the control-required target, the upper target and the lower target relative to the reference transmission axis. Then, the radar apparatus outputs the target information including the derived positions of the targets, to a vehicle controller, and the vehicle controller determines, based on the target information acquired from the radar apparatus, whether or not the vehicle control is required.
On the other hand, there is a need to transmit the transmission waves in a relatively wide range in a width direction of the host vehicle. In order to meet the need, at least two antennas each for the three types, 6 antennas in total, are required to cover a wider angle range in the width direction.
However, in a case where the radar apparatus includes the three types of the horizontal antenna, the upper antenna, and the lower antenna to realize “wider” angle to derive the three types of targets by transmitting the transmission waves in the relatively wide range in the width direction of the host vehicle, a relatively large area is required for installation of those antennas. Accordingly, a size of the radar apparatus becomes large so that the radar apparatus can be mounted only on limited portions of a vehicle. Moreover, the more the transmitting antennas are used, the more parts are required for the transmitting antennas, such as an electric power supply circuit that supplies transmission signals to those antennas. Thus, parts cost for manufacturing the radar apparatus also increases so that miniaturization and price reduction of the radar apparatus will be difficult.